The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a wireless device such as a user equipment (UE) 150a is wirelessly connected to a radio base station (RBS) 110a commonly referred to as an evolved NodeB (eNodeB), as illustrated in FIG. 1a. Each eNodeB 110a, 110b serves one or more areas each referred to as cells 120a, 120b, and are connected to the core network. In LTE, the eNodeBs 110a, 110b are connected to a Mobility Management Entity (MME) (not shown) in the core network. A positioning server 140, also called a location server, in the control plane architecture in FIG. 1a is connected to the MME. The positioning server 140 is a physical or logical entity that manages positioning for a so called target device, i.e. a wireless device that is being positioned. The positioning server is in the control plane architecture also referred to as an Evolved Serving Mobile Location Center (E-SMLC). As illustrated in FIG. 1a, the E-SMLC 140 may be a separate network node, but it may also be a functionality integrated in some other network node. In a user plane architecture, the positioning is a part of a Secure User Plane Location (SUPL) Location Platform (SLP). The positioning server may be connected to radio network nodes via logical links while using one or more physical connections via other network nodes e.g., the MME. A Network Management (NM) or Operations and Maintenance (O&M) node 141 may be provided to perform different network management operations and activities in the network.
The possibility of identifying user geographical location in a network has enabled a large variety of commercial and non-commercial services, e.g., navigation assistance, social networking, location-aware advertising, emergency calls, etc. Different services may have different positioning accuracy requirements imposed by an application. In addition, some regulatory requirements on the positioning accuracy for basic emergency services exist in some countries, e.g., FCC E911 in the U.S.
Three key network elements in an LTE positioning architecture are a Location Services (LCS) Client, an LCS target and an LCS Server. The LCS Server is a physical or logical entity managing positioning for a LCS target device by collecting measurements and other location information, assisting the terminal in measurements when necessary, and estimating the LCS target location. The LCS Client is a software and/or hardware entity that interacts with the LCS Server for the purpose of obtaining location information for one or more LCS targets, i.e. the entities being positioned. The LCS Clients may reside in the LCS targets themselves. An LCS Client sends a request to the LCS Server to obtain location information, and the LCS Server processes and serves the received requests and sends the positioning result and optionally a velocity estimate to the LCS Client. A positioning request can be originated from a terminal or the network.
Two positioning protocols operating via the radio network exist in LTE, LTE Positioning Protocol (LPP) and LPP Annex (LPPa). The LPP is a point-to-point protocol between a LCS Server and a LCS target device, used in order to position the target device. LPP can be used both in the user and control plane, and multiple LPP procedures are allowed in series and/or in parallel thereby reducing latency. In the control plane, LPP uses RRC protocol as a transport.
LPPa is a protocol between eNodeB and LCS Server specified mainly for control-plane positioning procedures, although it still can assist user-plane positioning by querying eNodeBs for information and eNodeB measurements. Secure User Plane (SUPL) protocol is used as a transport for LPP in the user plane. LPP has also a possibility to convey LPP extension messages inside LPP messages, e.g., currently Open Mobile Alliance (OMA) LPP extensions (LPPe) are being specified to allow, e.g., for operator- or manufacturer-specific assistance data or assistance data that cannot be provided with LPP or to support other position reporting formats or new positioning methods. LPPe may also be embedded into messages of other positioning protocol, which is not necessarily LPP.
A high-level architecture, as it is currently standardized in LTE, is illustrated in FIG. 2, where the LCS target is a terminal 200, and the LCS Server is an E-SMLC 201 or an SLP 202. In the figure, the control plane positioning protocols with E-SMLC as the terminating point are shown by arrows 203, 204 and 205, and the user plane positioning protocol is shown by arrows 206 and 207. The SLP 202 may comprise two components, SUPL Positioning Centre (SPC) and SUPL Location Centre (SLC), which may also reside in different nodes. In an example implementation, the SPC has a proprietary interface with the E-SMLC 201, and an Llp interface with SLC, and the SLC part of SLP communicates with a PDN-Gateway (P-GW) (not shown) and an external LCS Client 208.
Additional positioning architecture elements may also be deployed to further enhance performance of specific positioning methods. For example, deploying radio beacons is a cost-efficient solution which may significantly improve positioning performance indoors and also outdoors by allowing more accurate positioning, for example, with proximity location techniques.
UE positioning is a process of determining UE coordinates in space. Once the coordinates are available, they may be mapped to a certain place or location. The mapping function and delivery of the location information on request are parts of a location service which is required for basic emergency services. Services that further exploit a location knowledge or that are based on the location knowledge to offer customers some added value are referred to as location-aware and location-based services. The possibility of identifying a wireless device's geographical location in the network has enabled a large variety of commercial and non-commercial services, e.g., navigation assistance, social networking, location-aware advertising, and emergency calls. Different services may have different positioning accuracy requirements imposed by an application. Furthermore, requirements on the positioning accuracy for basic emergency services defined by regulatory bodies exist in some countries. An example of such a regulatory body is the Federal Communications Commission regulating the area of telecommunications in the United States.
In many environments, a wireless device position can be accurately estimated by using positioning methods based on Global Positioning System (GPS). Nowadays, networks also often have a possibility to assist wireless devices in order to improve the device receiver sensitivity and GPS start-up performance, as for example in an Assisted-GPS (A-GPS) positioning method. GPS or A-GPS receivers, however, may not necessarily be available in all wireless devices. Furthermore, GPS is known to often fail in indoor environments and urban canyons. A complementary terrestrial positioning method, called Observed Time Difference of Arrival (OTDOA), has therefore been standardized by 3GPP. In addition to OTDOA, the LTE standard also specifies methods, procedures, and signaling support for Enhanced Cell ID (E-CID) and Assisted-Global Navigation Satellite System (A-GNSS) positioning. In future, Uplink Time Difference of Arrival (UTDOA) may also be standardized for LTE.
OTDOA Positioning
With OTDOA, a wireless device such as a UE measures the timing differences for downlink reference signals received from multiple distinct locations. For each measured neighbor cell, the UE measures Reference Signal Time Difference (RSTD) which is the relative timing difference between a neighbor cell and the reference cell. As illustrated in FIG. 3, the UE position estimate is then found as the intersection 430 of hyperbolas 440 corresponding to the measured RSTDs. At least three measurements from geographically dispersed RBSs 410a-c with a good geometry are needed to solve for two coordinates of the UE. In order to find the position, precise knowledge of transmitter locations and transmit timing offsets is needed. Position calculations may be conducted, for example by a positioning node such as the E-SMLC or the SLP in LTE, or by the UE. The former approach corresponds to the UE-assisted positioning mode, and the latter corresponds to the UE-based positioning mode.
In UTRAN Frequency Division Duplex (FDD), an SFN-SFN type 2 measurement (SFN stands for System Frame Number) performed by the UE is used for the OTDOA positioning method. This measurement is the relative timing difference between cell j and cell i based on the primary Common Pilot Channel (CPICH) from cell j and cell i. The UE reported SFN-SFN type 2 is used by the network to estimate the UE position.
Positioning Reference Signals
To enable positioning in LTE and facilitate positioning measurements of a proper quality and for a sufficient number of distinct locations, physical signals dedicated for positioning, such as positioning reference signals (PRS), have been introduced, and low-interference positioning subframes have been specified in 3GPP. PRS are transmitted from one antenna port R6 according to a pre-defined pattern, as described in more detail below.
A frequency shift, which is a function of a Physical Cell Identity (PCI), can be applied to the specified PRS patterns to generate orthogonal patterns and model an effective frequency reuse of six, which makes it possible to significantly reduce neighbor cell interference on the measured PRS and thus improve positioning measurements. Even though PRS have been specifically designed for positioning measurements and in general are characterized by better signal quality than other reference signals, the standard does not mandate using PRS. Other reference signals, e.g., cell-specific reference signals (CRS) may also be used for positioning measurements.
PRS are transmitted according to a pre-defined pattern and following one of the pre-defined PRS configurations. PRS are transmitted in pre-defined positioning subframes grouped by a number N_prs of consecutive subframes, i.e. one positioning occasion, as illustrated in FIG. 4. Positioning occasions occur periodically with a certain periodicity of N subframes, corresponding to a time interval T_prs between two positioning occasions. The standardized time intervals T_prs are 160, 320, 640, and 1280 ms, and the number of consecutive subframes N_prs are 1, 2, 4, and 6. Each pre-defined PRS configuration comprises PRS transmission bandwidth, N_prs and T_prs.
OTDOA Assistance Information
Since for OTDOA positioning PRS signals from multiple distinct locations need to be measured, the UE receiver often will have to deal with PRS that are much weaker than those received from the UE's serving cell. Furthermore, without approximate knowledge of when the measured signals are expected to arrive in time and what is the exact PRS pattern used, the UE would need to do signal search within a large window, which would impact the time and accuracy of the measurements as well as the UE complexity. To facilitate UE measurements, assistance information, also referred to as assistance data, is transmitted to the UE, which includes e.g. reference cell information, a neighbor cell list containing PCIs of neighbor cells, the number of consecutive downlink subframes N-prs, PRS transmission bandwidth, and frequency.
The assistance information is signaled over LPP from the positioning server, e.g., an E-SMLC in the control plane for an LTE system, to the UE.
OTDOA Inter-Frequency Measurements and Measurement Gaps
In LTE OTDOA, the UE measures Reference Signal Time Difference (RSTD) which has been defined in the standard as the relative timing difference between cell j and cell i, defined as TSubframeRxj−TSubframeRxi, where: TSubframeRxj is the time when the UE receives the start of one subframe from cell j, TSubframeRxi is the time when the UE receives the corresponding start of one subframe from cell i that is closest in time to the subframe received from cell j. The reference point for the observed subframe time difference shall be the antenna connector of the UE. The measurements are specified for both intra-frequency and inter-frequency and conducted in the RRC_CONNECTED state.
The inter-frequency measurements, including RSTD, are conducted during periodic inter-frequency measurement gaps which are configured in such a way that each gap starts at an SFN and subframe meeting the following condition:SFN mod T=FLOOR(gapOffset/10);subframe=gapOffset mod 10;with T=MGRP/10, where MGRP stands for “measurement gap repetition period” and mod is the modulo function. The E-UTRAN is required according to the standard to provide a single measurement gap pattern with constant gap duration for concurrent monitoring of all frequency layers and Radio Access Technologies (RATs). Two configurations are according to the standard required to be supported by the UE, with MGRP of 40 and 80 milliseconds (ms), both with a measurement gap length of 6 ms. In practice, due to switching time, this leaves less than 6 but at least 5 full subframes for measurements within each such measurement gap.
In LTE, measurement gaps are configured by the network, i.e. the eNodeB, to enable measurements on different LTE frequencies and/or different RATs such as e.g., UTRA, GSM and CDMA2000. A measurement is configured using the Radio Resource Control (RRC) protocol to signal a measurement configuration to the UE. The gap configuration is signaled to the UE as part of the measurement configuration. Only one gap pattern can be configured at a time. The same pattern is used for all types of configured measurements, e.g. inter-frequency neighbor cell measurements, inter-frequency positioning measurements, inter-RAT neighbor cell measurements, inter-RAT positioning measurements, etc.
In multi-carrier LTE, the inter-frequency measurement gaps are so far intended mainly for performing cell identification and mobility measurements, such as Reference Signal Receiver Power (RSRP) and Reference Signal Received Quality (RSRQ). These measurements require a UE to perform measurements over the synchronization signals, i.e., the primary synchronization signals (PSS) and secondary synchronization signals (SSS), and cell-specific reference signals (CRS) to enable inter-frequency handover and enhance system performance. Synchronization signals are transmitted over 62 resource elements in the center of the allocated bandwidth in subframes 0 and 5. The PSS is transmitted in the last OFDM symbol and the SSS is transmitted in the second to last OFDM symbol of the first slot of a subframe. CRS symbols are transmitted every subframe and over the entire bandwidth according to one of the standardized time-frequency patterns. Different cells can use 6 different shifts in frequency, and 504 different signals exist. With two transmit (TX) antennas, the effective reuse for CRS is three.
As can be seen from the above, both synchronization signals and CRS are transmitted relatively often, although PSS and SSS are transmitted less frequently than CRS. This leaves enough freedom when deciding the exact timing of measurement gaps so that a gap can cover enough symbols with the signals of interest, i.e., PSS/SSS and/or CRS. With a 6 ms measurement gap, at most two SSS and two PSS symbols are possible with very precise timing, while capturing one SSS symbol and one PSS symbol is possible almost without any timing restriction on the measurement gaps since the minimum required effective measurement time is 5 ms on average.
In LTE OTDOA, the network, i.e. the eNodeB, can signal a list of cells operating on up to three frequency layers, including the serving cell frequency. The 3GPP RAN4 requirements for RSTD inter-frequency measurements are defined for two frequency layers, including the serving cell frequency. Furthermore, the measurement gaps are to be defined such that they do not overlap with PRS occasions of the serving cell layer, which would otherwise increase the effective measurement time for both the serving and the inter-frequency cell. Since the measurement gaps configured for the UE are used for RSTD measurements and also for mobility measurements, it has been agreed that the pre-defined “Gap Pattern #0”, which specifies relatively dense and frequent measurement gaps, can be used only when inter-frequency RSTD measurements are configured. According to the pre-defined Gap Pattern #0 a measurement gap of 6 ms occurs every 40 ms.
As mentioned above, the measurement gaps to be applied by the UE are configured by the eNodeB over RRC. However it is the positioning server, e.g. E-SMLC, which is aware of whether and when the UE will conduct positioning inter-frequency measurements such as e.g., inter-frequency RSTD or inter-frequency E-CID and this information is transmitted to the UE transparently via the eNodeB. Thus, to be on the safe side the eNodeB may always configure UEs for the worst case, i.e. for the 40 ms measurement gap according to the Gap Pattern #0, even when the UEs measure only on intra-frequency cells. This is a severe restriction on the network in that it reduces the amount of radio resources available for intra-frequency measurements and it leads to an inefficient measurement procedure.